The present invention generally relates to a method of seismic exploration. In particular, a novel method of obtaining a vertical seismic profile of the earth's subterranean formations surrounding a well borehole is provided.
In vertical seismic profile exploration, seismic energy is periodically imparted into the earth's subterranean formations with a surface source at or near a well borehole. The seismic waves thus generated can be detected along the length of the well borehole with a sonde which develops signals representative of the earth's response to the imparted seismic energy. The response signals can be employed to produce a vertical seismic profile of the earth's subterranean formations surrounding the well borehole.
Various vertical seismic profiling techniques are presently available; however, such techniques have generally been limited to employing compressional wave seismic sources and sondes having vertically oriented geophones to obtain compressional vertical seismic profile data. More recently, DiSiena, et al., 51st Annual International SEG Meeting (1981) V. 4 pages 1990-2011, have developed a vertical seismic profiling technique employing impulsive seismic sources, which impart both shear wave and compressional wave seismic energy, e.g., a Dinoseis (a trademark of ARCO Oil and Gas Company) source, and a sonde having three component geophones adapted to detect motion along vertical (z) and horizontal (x-y) axes to conduct vertical seismic profile exploration. In such case, shear wave and compressional wave vertical seismic profile data can be obtained for the formations surrounding the well borehole from the imparted shear and compressional wave seismic energy, i.e., each vertical seismic profile display includes both shear wave and compressional wave responses of the earth. Others, such as Devaney, et al., European Patent Application No. 148,076, have disclosed a vertical seismic profiling technique wherein the compressional and shear components can be related analytically to the imparted wavefield using a sonde with at least two geophones both aligned in the source-borehole plane, more precisely the plane defined by the source and the well borehole axis, and certain knowledge of the acoustic properties of the formation surrounding the well borehole. Still others, such as Hardage, "Vertical Seismic Profiling: Part A: Principles," Geophysical Press, 1983, page 413, have proposed separating the compressional and shear components of a measured wavefield by the expedient of mathematical rotations. However, in locales having complex subsurface formations where converted modes of seismic energy can be present, it is believed such techniques will not reliably separate the compressional and shear components of the wavefield. Whereas, the present invention is directed to a method of vertical seismic profiling to excite and extract both compressional and shear seismic responses of the earth's formations utilizing a horizontally directed swept frequency vibrator and a multicomponent sonde.